I bought a KAB4 burner. There are cheaper propane burners out there, but they are slower/lower BTU. The KAB4 is the same base banjo burner in a lot of the high end burners - like the edelmetal bru or the blitchmann hellfire - just with a different frame and at about half the price.

Since doing the kettle and the burner would probably hurt the budget too much, another option would just be to go with a turkey fryer setup. It'll be a smaller/crappier kettle and a worse burner, but something like this would get you going on larger batches for about $50